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24 Chapter 2 Scalability and Availability Overview
For JRun and ColdFusion web applications, it is particularly important that the servers
remain as highly available and responsive as the web server and other dependent servers.
JRun and ColdFusion process requests sent to them from the web server. Upon
successfully processing the application logic, JRun and ColdFusion return the results to
the web server, which in turn returns an HTML response to the browser.
Availability and reliability are concerned with keeping the relevant servers that provide
services to your web application available at all times. However, if a server on which your
site depends becomes unavailable, you must have a sound redundancy scheme to make
certain that your site remains available. As your organization moves into an e-business
paradigm, you must plan, design, and implement load-balancing and failover strategies
that guarantee that your servers will remain operational.
If servers employ a good strategy for load balancing and failover, they provide high
availability and reliability to their users. In fact, Internet Service Providers (ISPs) that
host commercial websites and offer 24x7 technical support as a competitive service
differentiator typically specify in written service-level agreements a percentage of time
that they guarantee a website will be available. If the ISP has a sound scalability and
failover strategy in place, this figure is usually in the range of 99% or better.
Common failures
Following are typical types of failures that can negatively impact your web application’s
availability and reliability:
•
Hardware failures — while less common than software failures, hardware failures do
occur, and can include crashed hard drives, blown processors, and corrupted network
cards. Diagnosing and fixing these issues can be a lengthy endeavor because of time
spent getting parts and performing labor. If your web application is mission-critical,
you should ensure a sound hardware redundancy strategy to avoid costly downtime.
A sound strategy includes a minimum of two, but preferably three, web servers.
•
Software failures — the software failures that most affect a web application involve
the web server’s operating system, the web server software itself, or the web
application software. If the operating system crashes or becomes corrupt, the web
server cannot function properly (or perhaps at all), compromising your web
application’s availability, reliability, and performance. Similarly, if the web server
software crashes or acts erratically, it will probably cause the web server to stop
running. Preparing for software failures is difficult, but if you have mirrored
secondary hardware systems in place to account for failures, you’ll minimize your web
application’s downtime.
•
Server failures — other servers on which your web application depends can also fail,
causing downtime or diminished capabilities on your site. For example, for
distributed applications, a proxy server might go down, causing requests for your web
application’s services to go unanswered. Or the database server might crash, making it
impossible for users to submit or retrieve information from your database. Or a mail
server might go down, making it impossible for your users to successfully send mail
to you. Ensure that your organization’s IT architecture includes network monitoring
and notification software that can quickly report on the general health of your
network and alert you about any failed servers.